Extractive distillation process



Feb. 23, 1954 H. GREKEL ETAL EXTRACTIVE DISTILLATION PROCESS Filed Oct.28. 1949 WATER FEED Fig!

. $5: EQHESE Howard Grekel Benjamin 8. Pace John D.lrelund INVENTORSHEAT INPUT- sTL/La. BOTTOMS Fig.2

ATTORNEY.

Patented Feb. 23, 1954 EXTRACTIVE DISTILLATI-ON PROCESS Howard Grekel,Benjamin S. Pace, and John D. Ireland, Tulsa, Okla., assignors toStanolind Oil and Gas Company, Tulsa, Okla., a corporation of DelawareApplication October 28, 1949, Serial N 0. 124,228

Claims.

The present invention relates. to a novel improvement in the art ofseparating the components of various azeotropic or similar mixtures bymeans of extractive distillation. More particularly, it pertains to amethod by which ideal conditions for the separation of the individualcomponents of such mixtures are achieved.

In separating mixtures of close boiling compounds or azeotropic mixturesinto their individual components by means of extractive distilq lation,there exist both maximum and minimum heat input limits within which thecolumn must be operated to secure a satisfactory separation. Primaryfactors affecting the maximum and minimum heat input are the compositionof the feed mixture, the degree of separation desired, and theconcentration of extractive agent maintained in the liquid on theplates. The minimum heat input for an extractive distillation isanalogous to the minimum reflux ratio concept for conventionaldistillation that a certain minimum input of heat (as indicated by theratio of vapor rising in the column to liquid flowing down the column)is required to secure the necessary fractionating action. The maximumlimit on heat input to extractive distillation columns results from thefact that as heat input is increased, the concentration of theextractive agent in the liquid on the plates decreases. This results indecreased relative volatility of the key components with correspondingincreased difficulty in separation.

It is an object of our invention to provide an improved method forseparating various azeotropic or close boiling mixtures into theirindividual components by means of extractive distillation of suchmixtures, this object being achieved by adjusting conditions to approachthe optimum liquid to vapor ratios (hereinafter referred to as L./V.) inboth the washing and the stripping sections of the column. It is afurther object of our'invention to control the L./V. in th stripping andwashing sections of the column by effecting condensation of a portion ofthe vapors present in the column substantially at the point of feedintroduction. Two preferred methods of accomplishing this object consistof either regulating thtemperature at which the liquid feed isintroduced into the column or withdrawing heat from the column in theform of vapors substantially at the level at which the feed isintroduced, condensing these vapors by passing them in indirect heatexchange relationship with the extraction solvent or other coolingmedium, and returning the resulting condensate to the column at theapproximate point of vapor withdrawal. A still further object of ourinvention is to effect a substantial savings in the numher of. t al plats. r quired, by Iractionatine column em loye in extractive distilla i np= eration and, at the sametime, accomplish a sharp separation of thechemicals present in a given mixture.

We have found that for extractive distillation not only is there amaximum and minimum heat input to the column, but there are independentmaximum and minimum heat inputs for both the stripping and washingsections. In general, the, optimum heat input for the stripping sectionwill not correspond to the optimum heat input for the washing section,and we have found it desirable, in certain instances, to reduce the heatinput into the washing section in order to more closely approach theoptimum ratios of liquid to vapor for both sections. Thus, within thelimits. set by the system and the overall separation to be made, theeiiiciency of extractive distillation operations can be unexpectedly andmaterially improved by maintaining a high stripping rate (low L./V.) inthe stripping section and a high wash rate (high L./V.) in the washingsection of the column. The above-mentioned conditions may be achieved inat least two ways, (1) the feed may be introduced into the column at apoint intermediate the washing and stripping sections at a temperaturesufiiciently below its boiling point to effect a condensation of aportion of the vapors, i. e., preferably from about 10 to about 50 percent, rising into the upper section of the column, or (2) a limitedproportion of the total vapors from the chemical feed. tray may bewithdrawn from the column, condensed, for example, by passing the vaporsin indirect heat exchange relationship with cooling water or with theextrac-.. tion solvent, and returning the resulting con-. densate to thecolumn at the approximate point of withdrawal. By either method or bymeans of a combination of these methods, we have found it possible tooperate at the optimum L./V. in both sections. Observance of suchconditions re-. sults in a substantial decrease in the number of platesrequired in both sections. However, the mannerin which condensation ofthe feed tray or other vapors is carried out is not regarded asimportant and forms no part of our invention. While condensation of fromabout 10 to 50 per cent of such vapors is generally considereddesirable, the preferred range of vapor condensation to be eiiectedmight be even higher, On the other hand, we have found that by effectingeven a slight condensation (less than 10 per cent) of the tray vapors,conditions are provided inboth the washing and stripping sections of thecolumn which result in an improvement in the operating efficiency ofboth of said sections.

Any mixtures of close boiling compounds or azeotropic mixtures that maybe conveniently separated by means of extractive distillation the cnventio l man er an e s parate accordance with our inven ion and it stobe s ric ly under o d th the e pre sion 3 positions of such type.Typical of the various mixtures that may be separated into theirindividual organic compounds are ethanol-ethyl acetate-water,acetone-methanol-water, ethanolwashing sections to the number oftheoretical plates required for effecting separation of the componentsof such a system are shown in the table below. This relationship islikewise graphimethyl ethyl ketone-water, isopropanol-methyl 5 callypresentedinFig. 2.

TABLE I wAzenbge LIV. Average Theoretical Plates Required 86! on- 2%telitinlfate lb 'Bz'ms. Stripping Washing MoLemtPersemen sectionStripping Washing Total 84. 6 10.0 I s. s 11. 9. o 20. 0 82.3 7.0 6.2 as7.4 14.2 so. 5. 4 a 7 5. s 6.9 12. 2 7s. 1 4. 0 3. 4 4 7 s. 1 12. s 400-73.4 3.2 2.8 4.4 8.6 13.0

ethyl ketone-water, ethanol-isopropyl alcoholwater, and methyl ethylketone-ethyl acetatewater. Mixtures of the above type containing theindicated components in azeotropic proportions are encountered in theaqueous fraction resulting from the reaction of carbon monoxide withhydrogen in the presence of a promoted fluidized iron catalyst attemperatures of from 450 to 700 F. and pressures ranging from 150 toabout 450 p. s. i., and the process of our invention is particularlyapplicable to the separation of such mixtures into their respectivecompounds. Mixtures of water soluble chemicals derived from the directoxidation of hydrocarbons can likewise be eiiiciently separated inaccordance with the process of our invention.

Our invention may be further ililustrated by reference to Fig. 1 inwhich a preferred embodiment thereof is diagrammatically presented. Anaqueous mixture of chemicals to be separated is introduced in liquidform through line 2 into a fractionating column 4. The liquid in thebase of the column is vaporized by means of heat supplied from reboiler6 and extractive distillation is effected by countercurrently contactingthe rising vapors with a descending water stream introduced above thefeed entry point through line 8. The vapors passing upwardly through thedescending water stream are withdrawn through line [0 and condenser l2,a portion of the condensate being returned to the column through line l4while the balance is collected as distillate through line H). Duringdistillation, a portion of the vapors from the tray or plate on whichthe aqueous mixtures of chemicals is introduced is withdrawn throughline It and cooler l8 where it is condensed by heat exchange withsolvent water coming from line 20. The solvent water from cooler I8 thenpasses through line 8 and heater 22 after which it is introduced intocolumn 4. The condensate obtained in cooler i8 is conducted back throughline 24 to column 4 at the approximate point of vapor withdrawal. Thebottoms fraction is withdrawn through line 26.

Thus, for example, where it is desired to obtain from a mixturecontaining 17 mole per cent methyl ethyl ketone, 66 mole per centethanol, and 17 mole per cent water, a bottoms product containing 1 moleper cent of methyl ethyl ketone in ethanol and a distillate containing 1mole per cent ethanol in methyl ethyl ketone, this object may beaccomplished by introducing dilution water into the top of the column inan amount sufficient to yield a bottoms containing 90- mole per centwater. The relationship of heat input and the average L./V. in thestripping and It is to be noted from the above table, as well as in theaccompanying graph, that as heat input is increased, L./V. in bothsections decreases. The number of washing platesrequired reaches aminimum at L./V. of 4.7 whereas the number of stripping plates is stillslowly decreasing at L./V. of 3.2. The concentration of water in theplate liquid decreases as heat input is increased. At some heat inputbeyond 400 B. t. 11. per lb. of bottoms the number of plates required inthe stripping section also will begin to increase due to the decreasedrelative volatility resulting from the lessened concentration of waterin plate liquid. At heat inputs below 100 B. t. u./lb. of bottoms, platerequirements in both sections rapidly approach infinity. Thus, it may beseen that the minimum plate requirement for the wash section occurs at ahigher L./V. than the minimum plate requirement for the strippingsection. This principle can be utilized to economic advantage in othercases cited below.

The process of our invention is particularly adapted to extractivedistillation operations involving a very dilute feed and may be furtherillustrated by the specific example which follows.

Example A column charging a feed consisting of 2.5 mole per centethanol, 8.0 mole per cent methyl ethyl ketone, and the balance water,produced a distillate containing high purity methyl ethyl ketone and abottoms containing high purity ethanol. Under these circumstances, morethan enough water was present in the feed to permit an easy separationof methyl ethyl ketone from ethanol. In the interest of economy,therefore, it was desired to hold at a minimum the volume of extractiveagent (water) introduced at the top of the column to provide thenecessary washing action. In such case it has been found highlydesirable to cool the feed below its boiling point in order to reduce toa minimum the vapor entering the upper section. Thus, with a column operating at a feed rate of 10,000 lbs. per hour and having a reboilerheat input of B. t. u./lb. of bottoms and a feed of the abovecomposition, the total vapor rising in the base of the column was 44moles per hour. If the feed were introduced at its boiling point, 202 R,the vapor rising into the upper section would amount to some 64 molesper hour. By cooling the feed to the feed plate temperature, F., thevapor passing into the upper section was reduced by some 30 per cent to44 moles per hour. For a constant water input at the top of the washsection of 740 lbs. per hour, the cooling of feed permitted a saving ofsome five actual plates due to the '5 higher L./V. in the washingsection. It is also apparent that cooling the feed in this instancereduced the diameter of column required.

The advantages afforded by Operating in accordance with the teachings ofour invention are further illustrated by the data appearing in the tablebelow. In case A, conventional extractive distillation was employed,while in case B, the improved extractive distillation technique of ourinvention, as generally illustrated in the abovementioned drawing, wasutilized. The feed mixture selected for treatment in cases A and Bconsisted of ethanol and isopropyl alcohol in a ratio of 4:1 togetherwith azeotropic water. A separation of this type is difiicult to effect,requiring a high concentration of water (95 mole per cent minimum) inthe column and a large number of plates. In both cases the chemical feedwa introduced at the thirty-eighth plate which, in our opinion, is theoptimum feed point location. The distillate obtained in each instancecontained less than 1 mole per cent ethanol in isopropyl alcohol and thebottoms contained 1 mole per cent isopropyl alcohol in ethanol. Arectifying section was employed to concentrate the distillate to theazeotropic composition. Dilution water was added to yield a bottomscontaining 1 mole per cent total chemicals and net heat input was set at150 B. t. u. per 1b. of bottoms. Under these conditions, case A requireda total of '77 actual plates to effect the separation desired. In caseB, by condensing 25 per cent of the feed plate vapor and returning theresulting condensate to the approximate point of vapor withdrawal, thewash plate requirements were decreased by per cent. An additionaladvantage secured by operating in accordance with case B is that thecold dilution water may be preheated by the condensing vapor whichsupplies about 25 per cent of the heat required to raise this stream tothe dilution water tray temperature. The specific results obtained ineach case appear as follows:

TABLE II From the foregoing it will be apparent that the processdescribed herein constitutes an important and distinct advance in theart of extractive distillation; and while the present invention has beendescribed with respect to its applicability to the separation of variousaqueous azeotropic mixtures of oxygenated organic chemicals, it may beapplied with equal success to nonaqueous azeotropic or other mixturesthat are difficult to separate into their individual components byordinary distillation methods. Thus, for example, the process of ourinvention may be employed in the separation of complex fractions ofpetroleum hydrocarbons whose components have small differences inboiling points; specifically our process may be utilized in theseparation of mixtures of diolefms, olefins, and parafiins which are notreadily separable by means of conventional fractional distillation.Other means may be employed to effect cooling of the feed plate vapors;for example, this object may be accomplished by inserting a cooling coilnear the feed plate whereby condensation of the vapors is achievedwithin the column.

We claim:

1. In a process for effecting a separation of close-boiling aqueousmixtures of water-soluble oxygenated organic compounds by means ofextractive distillation with water in a column having a stripping and awashing section therein whereby the relative volatilities of saidcompounds are modified, the improvement which comprises independentlyadjusting the liquid to vapor ratios of the components within saidsections and increasing the relative volatilities of said compounds insaid sections by introducing said mixture, at a temperaturesubstantially below its boiling point, into said column to effectcondensation of from about 10 to about 50 per cent of the vapors presentin said column substantially at the point of the feed inlet.

2. The process of claim 1 in which the watersoluble oxygenated organiccompounds to be separated are isopropyl and ethyl alcohols.

3. The process of claim 2 in which the watersoluble oxygenated organiccompounds to be separated are methyl ethyl ketone and ethyl alcohol.

4. In a process for efiecting separation of isopropyl alcohol from anaqueous mixture of ethyl alcohol by means of extractive distillationwith Water in a column having a stripping and a washing section thereinwhereby the relative volatilities of the aforesaid alcohols aremodified, the improvement which comprises independently adjusting theliquid to vapor ratios of the components within said sections andincreasing the relative volatilities of said alcohols in said sectionsby condensing a portion of the vapors present in the columnsubstantially at the point of the feed inlet.

5. In a process for effecting separation of methyl ethyl ketone from anaqueous mixture of ethanol by means of extractive distillation of waterin a column having a stripping and a washing section therein whereby therelative volatilities of the aforesaid ketone and alcohol are modified,the improvement which comprises independently adjusting the liquid tovapor ratios of the aforesaid compounds within said sections andincreasing the relative volatilities of said ketone and alcohol in saidsections by condensing a portion of the vapors present in the columnsubstantially at the point of the feed inlet.

HOWARD GREKEL. BENJAMIN S. PACE. JOHN D. IRELAND.

References Cited in the file of this patent UNITED STATES PATENTS NameDate Metzl July 21, 1942 Souders June 26, 1945 Gilliland et a1 May 8,1951 OTHER REFERENCES Number

5. IN A PROCESS FOR EFFECTING SEPARATION OF METHYL ETHYL KETONE FROM AN AQUEOUS MIXTURE OF ETHANOL BY MEANS OF EXTRACTIVE DISTILLATION OF WATER IN A COLUMN HAVING A STRIPPING AND A WASHING SECTION THEREIN WHEREBY THE RELATIVE VOLATILITIES OF THE AFORESAID KETONE AND ALCOHOL ARE MODIFIED, THE IMPROVEMENT WHICH COMPRISES INDEPENDENTLY ADJUSTING THE LIQUID TO VAPOR RATIOS OF THE AFORESAID COMPOUNDS WITHIN SAID SECTIONS AND INCREASING THE RELATIVE VOLATILITIES 